1. Field of the Invention
The present invention relates to a method for forming deep trench capacitors, and more specifically to a method for increasing capacitance of deep trench capacitors.
2. Description of the Related Art
Generally speaking, capacitors widely used in dynamic random access memory (DRAM) are formed by two conductive layers (electrode plate) having an insulation layer between. The ability to store electric charges of a capacitor depends on the thickness of the insulation layer, surface area of the electrode plate and the electrical characteristics of the insulation material. In recent developments for reducing sizes of semiconductor elements to enhance integration density, the area of memory cells in a memory must continuously be reduced to support a larger number of memory cells, thereby increasing density. Meanwhile, the electrode plates of a capacitor in a memory cell must contain sufficient surface area to store sufficient electrical charges.
Nevertheless, with sizes of elements continuously reduced, the trench storage node capacitance of DRAM is decreased as well. As a result, the storage capacitance must be increased to maintain good operating performance.
Consequently, it is necessary to develop a method for increasing the storage capacitance, for example, etching the semiconductor substrate to enlarge surface area of the bottom of the trench, and then forming a bottle-shaped trench capacitor. Currently, the above method is widely used for increasing the storage capacitance of DRAM.
FIGS. 1A to 1C are cross-sections illustrating the conventional process flow for forming a bottle trench. First, referring to FIG. 1A, a patterned pad layer 12 is formed on a silicon substrate 10. Then, the patterned pad layer 12 is used as an etching mask to etch the silicon substrate 10 by dry etching to form a trench 14 containing an upper portion 16 and a lower portion 18. The dimension 13xe2x80x2 of the trench 14 is formed.
Next, referring to FIG. 1B, a photoresist layer (not shown) covering the lower portion 18 of the trench 14 is deposited. A sacrificial layer 20 covering the upper portion 16 of the trench 14 and the pad layer 12 is deposited. Then, the photoresist layer (not shown) is removed and the sacrificial layer 20 on the pad layer 12 is removed by anisotropic etching later. Finally, the sacrificial layer 20 is formed in the upper portion 16 of the trench 14.
Subsequently, referring to FIG. 1C, the silicon substrate 10 uncovered by the sacrificial layer 20 of the lower portion 18 of the trench 14 is etched by isotropic wet etching using ammonia and diluted hydrogen fluoride to form the lower portion 22 of the bottle-shaped trench 14. The dimension 13 of the lower portion 22 is larger than the dimension 13xe2x80x2 of the upper portion 16 of the trench 14.
It is difficult to control the shape of the lower portion 22 of the trench 14 by the above method, and it results in increased instability and difficulty of the fabricating process.
In order to solve the conventional problems, an object of the invention is to provide a method for increasing the storage capacitance of a deep trench capacitor of a DRAM by enlarging surface area of the lower portion of the trench.
The invention forms a larger dimension of the trench at first to enlarge surface area of electrode plates of the capacitor, then narrows the dimension of the trench by forming an epitaxy layer on the sidewalls in the upper portion of the trench. Finally, the predetermined dimension is formed. The structure of the epitaxy layer is the same as the substrate, so that the electrical characteristics of the memory are maintained. The invention is formed as a bottle-shaped trench as well, thereby improving control of the shape of the bottom of the trench, further, increasing stability of the fabricating process.
The method for increasing the capacitance of the trench capacitors provided in the invention includes the following steps. First, a substrate is provided, and a pad structure is formed on the substrate. Next, a photoresist defining the deep trench is formed on the pad structure, and a deep trench is formed in the substrate. Subsequently, the photoresist is removed. A capacitor is formed in the lower portion of the deep trench. Next, a first insulation layer is formed on the capacitor, and an epitaxy layer is formed on the sidewalls of the deep trench above the first insulation layer as a liner to narrow the dimension of the deep trench. Finally, the first insulation layer uncovered by the epitaxy layer is removed.
A detailed description is given in the following embodiments with reference to the accompanying drawings.